Generally, the operation process of a refrigerant filling rotary compressor is that when the suction of the compressor is completed, the pressure in the compression chamber of the compressor is lower than that in the refrigerant injecting mouth. Thus, the injection valve is opened in one way to compress the injected gas in the compression chamber. As the piston moves, the volume of the compression chamber gradually decreases, and then the gas pressure therein gradually rises. When the pressure in the compression chamber equals to that in the refrigerant injecting mouth, the injection valve is closed. As the piston further moves, the volume of the compression chamber further decreases, and when the gas pressure therein is slightly higher than the exhaust pressure, the exhaust valve of the compressor is opened to exhaust the gas.
However, the ordinary refrigerant filling rotary compressors have the following defects: due to the presence of the injection valve and the filling mouth, high pressure gas filled in the space of the injection valve and the filling mouth cannot be further compressed and exhausted when the piston moves to the filling mouth. In such a case, an extra clearance volume of the compressor is formed and termed a clearance volume formed by the injection valve, thereby affecting the performance of the compressor. In addition, when the piston moves to the filling mouth, it is possible that the unexhausted gas which is being compressed in the compression chamber may leak into the suction chamber.